1. Field of the Invention
The present invention relates to a microlens array, a display element provided with microlenses, and a display device provided with such a display element.
2. Description of the Related Art
Projectors using liquid crystal display (LCD) panels, digital mirror devices (DMD), and other light valves are currently the focus of intensive development. Projectors include data projectors mainly for monitor display of personal computers, front projectors or rear projectors used mainly for home theater and other AV applications, and rear projectors for television applications. Further, they are divided into one- to three-plate types according to the number of light valves used. For example, there are a single-plate type using one light valve and a three-plate type using three light valves corresponding to the three colored lights of red (R), green (G), and blue (B). Further, light valves includes both transmission types and reflection types.
FIG. 1 is a view of an example of the configuration of a liquid crystal display panel in a projection type liquid crystal display device. The liquid crystal display panel shown in FIG. 1 is provided with a pixel electrode substrate 140B and a counter substrate 140A arranged facing an incident surface side of light of the pixel electrode substrate 140B via a liquid crystal layer 145.
The pixel electrode substrate 140B has a glass substrate 148, a plurality of pixel electrode portions 146 laminated on the incident surface side of light of this glass substrate 148, and a plurality of black matrix portions 147. The pixel electrode portions 146 and the black matrix portions 147 are arranged two-dimensionally. Each pixel electrode portion 146 is constituted by a transparent member having conductivity. Each black matrix portion 147 is formed between adjacent pixel electrode portions 146. Each black matrix portion 147 is shielded by for example a metallic film. A not illustrated switching element for selectively supplying voltage in accordance with an image signal to adjacent pixel electrode portions 146 is formed in each region shielded by a black matrix portion 147 on the glass substrate 148. As the switching element for supplying voltage to the pixel electrode portion 146, use is made of for example a thin film transistor (TFT).
The counter substrate 140A has, in order from the incident side of the light, a glass substrate 141, a microlens array 142, and cover glass 144 serving as the glass substrate having the microlens array 142 formed thereon. A resin layer 143 having a refractive index n1 is laminated between the glass substrate 141 and the microlens array 142.
The microlens array 142 is formed corresponding to individual pixels on the light incident side of the liquid crystal layer in order to raise the efficiency of utilization of the light of the light source. As shown in FIG. 1, due to the refraction of the microlenses 142M, the incident light from the light source does not strike regions shield by the black matrix portions 147, interconnect patterns, etc., but is condensed at only the openings of the pixel electrode portions 146.
The microlens array 142 has a plurality of microlenses 142M comprised of optical resin having a refractive index of 2n (>n1) and provided two-dimensionally corresponding to the pixel electrode portions 146. As shown in FIG. 1, the size of the microlenses 142M corresponds to the size of the pixels.
Each microlens 142M is shaped convexly at the incident side of the light, has a positive refraction power, and condenses light incident via the glass substrate 141 and the resin layer 143 toward the corresponding pixel electrode portion 146. The light condensed by the microlens 142M and incident upon the opening is used for image display.
Conventionally, each microlens 142 has been made aspherical in shape and incident light has been condensed to the pixel under conditions giving a spherical aberration of zero. For example, the microlens 142 is made aspherical in shape by cutting away part of its ellipsoid of revolution, hyperboloid of revolution, or the like. If using a microlens of such an aspherical shape, it becomes possible to condense the incident light to approximately a single point.
FIG. 2 is a view of the ray tracing of a microlens 142M having the ellipsoid of revolution. As shown in FIG. 2, the incident light L1 strikes the microlens 142M having the oval sphere. The emitted light L2 is substantially completely condensed at a focal point F. The same result is obtained for a microlens having the hyperboloid of revolution.
On the other hand, in the case of a microlens having a convex spherical surface, due to the spherical aberration, the emitted light cannot be condensed at a single point. It becomes a spot having a certain degree of spread, and the efficiency of the microlens is lowered.
Conventionally, as the method of formation of a microlens for a high definition liquid crystal display panel designed for a liquid crystal projector application, methods of using a quartz substrate or a neoserum substrate or other various glass substrates and forming the microlens by wet etching, photo polymerization, dry etching, etc. have been put into practical use.
In the etching process, the glass substrate is etched from a minute opening of a resist covering the surface of the glass substrate to form a concave spherical surface. A transparent resin having a different refractive index from that of the glass substrate is filled in this concave spherical surface to form the microlens.
In photo polymerization, ions are isotropically diffused into the glass from one point of the surface of the glass substrate to locally change the refractive index and form the microlens.
Wet etching is isotropic etching. Under actual circumstances, there is only the example of a microlens of a rotating symmetrical shape (for example a spherical shape). A lens power is produced by the refractive index difference Δn between the resin and the glass substrate to impart a lens action. In this case, since the etching of the substrate is isotropic, it can only form a spherical structure or a cylindrical surface or other rotating symmetrical shape. Under actual circumstances, the spherical structure is the general structure.
In dry etching, semiconductor formation technology is applied to also form aspherical surface shapes other than spherical surfaces.
Photo polymerization can be very precisely controlled. A process excellent in control of the lens shape can also be used to form a lens shape having a aspherical surface shape.
It is possible to form an aspherical surface by both photo polymerization and dry etching. A rotating ellipsoid can be relatively easily formed.
Since a microlens having an aspherical surface shape is used, there is no spherical aberration. Therefore, there is no dispersal of the light passed through the microlens due to spherical aberration, the light can be condensed with a sufficiently high efficiency within a range of an F number of the projection lens (angle at which the projection lens can be received), and the converging efficiency is more excellent than that of a spherical lens. Since an elliptic lens can be easily produced, conventionally a microlens having a rotating elliptic shape has been widely used.
Japanese Unexamined Patent Publication (Kokai) No. 9-127496 discloses a display device using microlenses of aspherical surface shapes of elliptic spherical surface shapes or rotating hyperboloid of revolution shapes.
Summarizing the problems to be solved by the present invention, in recent years, liquid crystal projectors have been made smaller in the size of the liquid crystal display panels and made higher in definition. When a liquid crystal display panel becomes small is size, as illustrated in FIGS. 3A and 3B, the pixel size is reduced in proportion to this. Therefore, the microlenses per se and the pitch arrangement become small. Along with this, it is also necessary to make the cover glass thinner.
FIG. 3A is a schematic view enlarging one pixel's worth of the liquid crystal display panel shown in FIG. 1. A microlens 142M condenses the incident light from a light source and emits it to an opening of a pixel electrode portion 146 surrounded by a black matrix portion 147, interconnect pattern, etc.
FIG. 3B shows a case where the pixel size and the pixel pitch are reduced. Along with the reduction of the pixel pitch, that is, the increase of the definition of the liquid crystal display panel, the microlens 142M per se must be similarly reduced. Due to this, it is necessary to shorten the focus of the microlens 142M and make the cover glass 144 extremely thin. Further, when the pixel size is reduced, the size of the opening of the pixel electrode portion 146 becomes relatively small, so means for projecting the light and maintaining a high illumination luminance on the screen for displaying the image is necessary.
Specifically, when the pixel pitch is 20 μm or more, if the aperture ratio is 40% or more and the focal length is 40 μm or more, a microlens 142M of an ellipsoid exhibits good performance, but when the pixel pitch becomes smaller, the aperture ratio becomes smaller along with that. Further, when the focal length is shortened, an ellipsoid microlens 142 has a larger variation in converging efficiency with respect to variation of the focal length and the thickness of the cover glass 144. Due to this, there is a tendency for the change in luminance to become greater.
Accordingly, when increasing the definition, if using an ellipsoid microlens 142M, it is necessary to finely control the focal length, so the cover glass 144 has to be made very thin. Due to this, at the stage of producing the cover glass 144, it is necessary to control the thickness of the resin and raise the polishing precision. These are technically difficult and lead to a rise of the costs.
For example, when trying to make the cover glass 144 thin to 30 μm or less in actual thickness, due to the difference of effective shrinkage or coefficient of thermal expansion of the optical resin constituting the microlens 142M, stress occurs and waviness and warping of the cover glass 144 occur. In general, cover glass 144 having an actual thickness of 20 μm is thought to be the limit of existing processing technology.